The present invention relates to positive displacement fluid pumps of the type which may be utilized in vehicle power steering systems, and more particularly, to such pumps which include a valve means to by-pass excess output fluid.
In vehicle power steering systems, positive displacement pumps are ordinarily employed for supplying pressurized fluid to a power steering control device, with the pump being driven by the vehicle engine and operating over a widely varying range of speeds.
Hydraulic pumps of the type to which the present invention relates include a housing defining a pumping chamber and a pumping element rotatably disposed in the pumping chamber. The housing defines an inlet port communicating with the expanding pumping pockets and an outlet port communicating with the contracting pumping pockets. The housing further includes a discharge port which may be connected to a fluid operated device, such as the power steering control unit.
In pumps of the type described, the volume of fluid flowing from the outlet port increases proportionally as the speed of the engine and pump increases. Therefore, in conventional power steering pumps, a by-pass valve is disposed in the flow path between the outlet port and the discharge port. The by-pass valve is ordinarily made responsive to the fluid pressure differential generated by a metering orifice positioned adjacent the discharge port, to establish a predetermined maximum flow rate from the discharge port. The excess fluid which is by-passed at higher pump speeds is typically recirculated toward the inlet port, either by dumping the excess fluid into the reservoir adjacent the inlet port or by directing the excess fluid into some type of inlet "header" which communicates with the inlet port. Frequently, such arrangements cause cavitation within the fluid or the trapping of air within the fluid because of turbulence caused by the mixing of inlet and by-pass fluids, which are at different pressures. In either case, the result is normally cavitation within the pumping element and excessively noisy operation of the pump.
Presently, there is a trend toward the use of "balanced" power steering pumps, i.e., pumps having a pair of diametrically opposed inlet ports, and a pair of diametrically opposed outlet ports with each pumping pocket expanding twice and contracting twice during each revolution, to balance loading on the input shaft. Providing sufficient filling of the pumping pockets to avoid cavitation is especially difficult in balanced pumps.
Accordingly, it is an object of the present invention to provide a pump of the type described in which excess fluid is redirected toward the inlet ports in a manner which improves filling of the pockets and reduces cavitation.
Another trend is to provide inlet ports on each axial end of the pumping element to improve filling and reduce cavitation. In one power steering pump which is presently commercially available, a pair of drilled passageways communicates excess fluid from the by-pass valve to one adjacent inlet kidney. Although such an arrangement may be satisfactory for some pump configurations, and under certain conditions, the use of drilled passageways to redirect or recirculate excess fluid clearly has serious limitations, especially in a pump which is balanced and double-end-fed.
Accordingly, it is another object of the present invention to provide a pump of the type described which is capable of directing by-pass fluid to a pair of inlet ports at one end of the pumping element, as well as to a pair of inlet ports at the opposite end of the pumping element.
It is a further object of the present invention to provide a pump which accomplishes the above-stated object without the necessity of an extremely complex, expensive casting or excessive and expensive machining.
The above and other objects of the invention are accomplished by the provision of an improved rotary pump of the type described above. The improvement comprises means defining a by-pass flow path of generally constant cross-sectional flow area communicating the excess fluid from a by-pass port adjacent the by-pass valve. The by-pass flow path includes a terminal portion disposed adjacent the inlet port and oriented generally axially to permit at least a portion of the excess fluid into the inlet port.